At the moment I am sitting at A's in both maths and further maths. I think the unit for my maths AS are C1, C2, D1 and for further maths they are combined as: FP1, M1, S1.

Is it possible for me to achieve A* in both? Or only in 1?

I heard I need an average of 90% in C3 and C4 for the maths A*, but I am unsure about further maths.

Yes, you are still OK.

The rules are:
For Maths at least 480/600 overall and 180 from C3 and C4.
For Further Maths at least 480/600 and at least 270/300 from your best A2 units (in your case the best 3 from S2, S3, M2 and FP2).
The system will automatically shift your applied units around to get you the best possible grades.